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The OneFS operating system architecture dictates that a file is written across multiple nodes in the cluster, and possibly multiple drives within a node. All read requests involve reading remote (and possibly local) data. When a read request arrives from a client, OneFS determines whether the requested data is in local cache. Any data resident in local cache is read immediately. If data requested is not in local cache, it is read from disk. For data not on the local node, a request is made from the remote nodes on which it resides. On each of the other nodes, another cache lookup is performed. Any data in the cache is returned immediately, and any data not in the cache is retrieved from disk.
When the data has been retrieved from local and remote cache (and possibly disk), it is returned back to the client.
The high-level steps for fulfilling a read request on both a local and remote node are:
On local node (the node receiving the request):
On remote nodes:
The following figures illustrate the steps of a read request on the local and remote nodes: